Band insulators with time reversal symmetry can be classified into normal insulators (NI), weak topological insulators (WTI) and strong topological insulators (STI) based on their Z2 topological indices. Changing Z2 indices requires closing and reopening the bandgap, and topologically distinct insulating phases are separated by a gapless Dirac or Weyl semimetal phase. The van der Waal layered material ZrTe5 is a prototypical example that the emergence of massive 3D Dirac fermions is due to its proximity to the STI-WTI phase boundary. In this talk, I will demonstrate that a STI-WTI topological phase transition can be induced by applying uniaxial stress to ZrTe5. I will also present another striking consequence of massive Dirac fermions in ZrTe5 - the observation of the anomalous Hall effect in this non-magnetic material, with a step-like angular dependence down an extremely low field of 0.03 T.